1. The Field of the Invention
The present invention is directed to methods for cleaning silicon wafers with an aqueous solution of hydrofluoric acid and hydriodic acid. Treatment of silicon wafers with an aqueous solution of hydrofluoric acid and hydriodic acid prevents copper deposition on the surfaces of the silicon wafers while minimizing the reoxidation of the silicon surfaces of the wafers.
2. The Relevant Technology
In the microelectronics industry, methods for cleaning silicon wafers are continually being developed and optimized to meet the stringent demands for wafers having clean and smooth surfaces. As the device features continuously decrease to the deep sub-micron region, the product yield and device performance become even more dependent on the wafer cleaning technology.
A clean, chemically stable, and atomically uniform silicon surface is an essential requirement prior to gate oxidation and silicon epitaxial growth in advanced ultra large-scale integration (ULSI) fabrication. It is well-known that metallic contamination on silicon surfaces can cause fatal effects on semiconductor devices. The metallic contamination on a silicon surface is preferably suppressed to less than 1E+10 atoms/cm.sup.2 in order to prevent defects. Wet chemical processing has been used as a major cleaning method in ULSI manufacturing; however, it has become increasingly sufficient to minimize metallic contamination using conventional cleaning methods such as RCA cleaning and hydrofluoric acid (HF) cleaning.
Hydrofluoric acid cleaning has been the main approach over the last few years for obtaining a clean, chemically stable, and atomically uniform silicon surface. After aqueous hydrofluoric acid treatment, a hydrogen-passivated bare silicon surface is obtained. Hydrofluoric acid treatment removes thermal and native oxides and is therefore an essential processing step of device fabrication and is a basic component of all kinds of cleaning procedures.
During hydrofluoric acid wafer cleaning, metals including noble metals, have been found deposited on wafer surfaces by oxidation-reduction reactions resulting in severely deteriorated device performances. Copper (Cu) deposition has been a particular problem.
The reaction in which a Cu.sup.2+ ion in a solution is metalized by taking electrons can be expressed by the following oxidation-reduction reaction equation: Cu.sup.2+ +2e.sup.- =Cu. The redox potential (E.degree.) of the metallization of a Cu.sup.2+ ion is 0.337 V. The electrons are considered to be supplied by the silicon.
The reaction in which silicon in an aqueous solution releases electrons can be expressed by the following equation: SiO.sub.2 +4H.sup.+ +4e.sup.- =Si+2H.sub.2 O. The redox potential for the reaction of silicon in an aqueous solution is -0.857 V. A Cu.sup.2+ ion, which has a higher redox potential than silicon, takes electrons, is reduced to metallic copper, and is deposited onto a silicon surface. Silicon, which features a lower redox potential than the Cu.sup.2+ ion, releases electrons and is oxidized to become silicon dioxide (SiO.sub.2). The copper deposition onto a silicon surface in the solution is essentially induced by the oxidation-reduction reaction between silicon and copper ions. Pits found where copper particles deposit on the silicon surface in a diluted hydrofluoric acid solution provides evidence of this SiO.sub.2 formation.
A pit produced when silicon dioxide (SiO.sub.2) formed in the oxidation-reduction reaction is etched away by a diluted hydrofluoric acid solution is referred to as a Metal Induced Pit (MIP). The mechanism of copper deposition onto silicon surfaces in solutions begins with Cu.sup.2+ ions in the vicinity of a silicon surface withdrawing electrons from the silicon and becoming precipitated in a form of metallic copper (Cu). It has been postulated that a nucleus of a copper particle is formed. As the copper nucleus adhering on the silicon surface features higher electronegativity than silicon, it attracts electrons from the silicon to become negatively charged. Other copper ions coming closer to the silicon surface gain electrons from the negatively-charged (electron-rich) copper nucleus and are precipitated around it. Accordingly, the copper nucleus grows into a larger particle on the silicon surface as more copper ions are precipitated. The silicon surface underneath the copper particles releases as many electrons as required by Cu.sup.2+ ions to be charged while SiO.sub.2 is thereby formed. In a diluted hydrofluoric acid solution, the formed SiO.sub.2 is etched away immediately and a pit is made.
The copper nucleus is considered to be made where a silicon surface is electrically active. Electron exchange between copper ions and silicon is more likely to take place at kinks, steps, and areas where halide ions are adsorbed because these areas are more electrically active than the hydrogen-terminated areas on a silicon surface. The promotion of copper deposition by a trace level of halogen ions in hydrofluoric acid solutions can be explained by this mechanism.
A typical pit formed from metallic impurities is about 0.1 .mu.m in diameter, which is also almost the same as the copper particle size. The depth of a typical pit from peak to valley is about 8 nm. The pit size can be fatal to device performance when it is considered that the thickness of a typical gate oxide is 8 to 15 nm.
Copper deposition has been prevented by the addition of strong oxidizing agents such as ozone and hydrogen peroxide. Strong oxidizing agents such as ozone and hydrogen peroxide are useful for the prevention of copper deposition; however, the surface of the silicon wafers are also thereby reoxidized.
There are no methods currently available which provide for the prevention of copper deposition and also the minimization of silicon surface reoxidation.